Brushless motor

ABSTRACT

A brushless motor with a rotor having a magnet and a magnetic return, the magnet and magnetic return being arranged to form a gap therebetween, and a stator having a coil disposed in the gap. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of co-pending application Ser.No. 10/125,809, filed Apr. 18, 2002, which claims priority under 35U.S.C. § 119(e) to provisional Application No. 60/291,704, filed May 16,2001 and provisional Application No. 60/354,792, filed Feb. 6, 2002, thecontents of which are expressly incorporated herein by referenced asthough fully set forth in full.

BACKGROUND

1. Field

The present invention relates to electrical machines, and moreparticularly, to brushless electric motors.

2. Background

Generally a brushless motor includes a rotor having a shaft with apermanent magnet affixed thereto. The brushless motor may also include astator having a hollow cylinder with ball bearing mounted on an interiorportion of the cylinder to rotatably support the rotor shaft. The statormay also include induction coil windings mounted on the case of themotor or interleaved with laminated iron rings. Current may be appliedto the stator windings to cause the magnet to rotate and thereby deliverrotary power to the rotor shaft.

Although brushless motors generally can be made to spin faster andhandle higher currents than brush motors, they have disadvantagesassociated with variations in the magnetic flux field at the magnets andthe laminations that comprise the magnetic return path, which may causecogging and concomitant vibration in the motor and associated structuresas well as energy losses in the laminations due to induced eddycurrents. Slotless brushless DC motors may exhibit reduced cogging butmay require that the wires comprising the stator be supported by a fixedlaminate structure thus having reduced energy conversion efficiency dueto induced eddy current losses. Energy losses due to heating of thelaminate structures found in brushless motors may negatively impact theefficiency of the motor and total power output. There is therefore aneed in the art for improvements in brushless motor design.

SUMMARY

In one aspect of the present invention, a brushless motor includes arotor having a magnet and a magnetic return, the magnet and magneticreturn being arranged to form a gap therebetween, and a stator having acoil disposed in the gap.

In another aspect of the present invention, a brushless motor includes arotor having a mounting surface, a magnet disposed on the mountingsurface, and a magnetic return surrounding the magnet to form a gaptherebetween, and a stator having a coil disposed in the gap.

In a further aspect of the present invention, a brushless motor includesa rotor having first and second concentric cylinders, and a magnetdisposed on an outer surface of the first cylinder, the second cylindersurrounding the first cylinder to form a cylindrical gap therebetween,and a stator having a free-standing cylindrical coil disposed in thegap.

It is understood that other aspects of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein is shown and described only exemplary embodimentsof the invention, simply by way of illustration. As will be realized,the invention is capable of other and different embodiments, and itsseveral details are capable of modifications in various respects, allwithout departing from the invention. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are illustrated by way of example, andnot by way of limitation, in the accompanying drawings in which likereference numerals refer to similar elements wherein:

FIGS. 1A and 1B are graphical illustrations exhibiting a plan view of apair of copper plates, precision cut for use in an exemplary brushlessmotor;

FIG. 2 is a graphical illustration of an elevation perspective view ofthe copper plate of FIG. 1A rolled into a hollow cylinder for use in anexemplary brushless motor;

FIG. 3 is a graphical illustration of an elevation perspective view ofthe copper plate of FIG. 1B rolled into a hollow cylinder for use in anexemplary brushless motor;

FIG. 4 is a graphical illustration of an elevation perspective view ofthe cylinder of FIG. 2 being inserted into the cylinder of FIG. 3 toform a cylindrical conductive coil for use in an exemplary brushlessmotor;

FIG. 4A is a graphical illustration of an enlargement a portion of FIG.4 illustrating detail of a wound glass fiber layer;

FIG. 5 is a graphic illustration of the interconnection of conductiveloops to form a continuous cylindrical conductive coil for use in anexemplary brushless motor;

FIG. 6 is a graphic illustration of a lateral cross-sectional view of anexemplary brushless motor;

FIG. 7 is a graphic illustration of a longitudinal cross-sectional viewillustrating the construction of a stator for an exemplary brushlessmotor;

FIG. 8 is a graphic illustration of a longitudinal cross-sectional viewof a rotor adapted for use with the stator of FIG. 7 in an exemplarybrushless motor; and

FIG. 9 is a graphic illustration of a transverse cross-sectional view ofthe rotor of FIG. 8 taken along section line 9-9 illustrating theposition of the stator coils within the cylindrical gap in the rotorwhen the stator and rotor are juxtaposed.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of thepresent invention and is not intended to represent the only embodimentsin which the present invention can be practiced. The term “exemplary”used throughout this description means “serving as an example, instance,or illustration,” and should not necessarily be construed as preferredor advantageous over other embodiments. The detailed descriptionincludes specific details for the purpose of providing a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout these specific details. In some instances, well known structuresand devices are shown in block diagram form in order to avoid obscuringthe concepts of the present invention.

In an exemplary brushless motor, a rotor includes a magnet which ispositioned concentrically with a stator coil and may rotate eitherinside the stator coil or outside of the stator coil. In either case, aniron magnetic return path is part of the rotor and rotates with themagnet. The construction of the rotor and stator coil may take onvarious forms depending on the specific application and the overalldesign constraints. By way of example, the rotor may be implemented witha metal shaft coupled to a rotor cap. A mounting surface carrying themagnet may be coupled to the rotor cap and arranged concentrically withthe shaft. The mounting surface can be an iron cylinder or any othersimilar structure known in the art. A magnetic return, which may also bean iron cylinder or similar structure, surrounds the mounting surface tocreate a magnetic air gap between the magnet and the magnetic return.The stator coil can be positioned in the magnetic air gap. A statorfaceplate supporting the stator coil may be formed with a hollowcylinder for rotatably supporting the rotor shaft with ball bearings orbushings. The shaft, mounting support, and magnetic return rotate aroundand within the stator coil during operation. In at least one embodimentof the brushless motor, the magnetic return can be inserted over statorcoil.

The stator coil can be constructed in a variety of ways. A free-standingstator coil is well suited for motor applications using a rotor with amagnetic return. A free-standing stator coil includes windings that arestructurally self sufficient. The windings of the free-standing coil donot need to be wound on the case of the motor or interleaved withlaminated iron rings. An exemplary free-standing coil is shown in FIGS.1-5. Referring to FIGS. 1A and 1B, the exemplary free-standing coil canbe constructed from a thin pair of nearly mirror image, electricallyconductive and precision-machined pieces of bare sheet metal plates 10and 12. The plates 10 and 12 may be made of tempered copper grade 110with each plate precision cut in a pattern to produce a series ofgenerally parallel conductive bands 18 and 22, with each band beingseparated from the other by an elongated cutout such as cutout 14 of theplate 10 and cutout 16 of the plate 12. The cutouts prevent electricalcontact between neighboring bands such as bands 18 and 20 of the plate12 and bands 22 and 24 of the plate 10. In the described exemplaryembodiment, the width of a cutout is about 1-1.5 times the conductorthickness. A cutout width of about 1-1.5 times the conductor thicknessreduces the electrical resistance over conventional approaches.

Each copper plate 10 and 12 is commonly 2 inch by 3 inch (approximately5 cm times 7.5 cm) and has a thickness of about 0.005 inch (0.12 mm).Other dimensions and materials may be used to manufacture the conductiveplates 10 and 12 depending on the particular application. The desiredpattern can be achieved by precision cutting the plates by chemicalmachining. The desired pattern can be machined by alternate techniquessuch as water jet cutting, laser cutting, electron beam cutting,punching, progressive die or other conventional machining methods.

The copper plate 10 includes carrier strips 26 and 28 on each edge, andthe copper plate 12 includes carrier strips 30 and 32 on each edge. Thecarrier strips support the conductive bands at each end and aresubsequently removed as explained below. The pattern also includes aseries of relatively small holes such as holes 34 and 36 of plate 10 andholes 38 and 40 of plate 12, one on each end of a conductive band. Anexemplary diameter for each hole is about 0.25 mm. The total number ofholes on each side is generally equal to the number of conductive bands.It will be appreciated that a stator comprising an induction coil ofthis type may be constructed from plates having less or more conductivebands and holes depending on the particular brushless motor operationalrequirements.

Plate 10 is rolled into a thin-walled hollow cylindrical shape such ascylinder 42, of FIG. 2. Plate 12 is also rolled into a thin-walledhollow cylindrical shape such as cylinder 44, of FIG. 3, but with itspattern of conductive bands and cutouts specifically oriented to createa near mirror image of the pattern of conductive bands and cutouts ofplate 10. An exemplary illustrative diameter of cylinder 42 is about0.510 inch (apx 2 cm) and an exemplary illustrative diameter of cylinder44 is about 0.520 inch (apx 2 cm). Cylinder 42 is formed with a slightlysmaller diameter to allow subsequent axial alignment of the same intocylinder 44 to form a conductive induction coil. For this reason,cylinder 44 will hereafter be referred to as outer cylinder 44 andcylinder 42 will respectively be referred to as inner cylinder 42. Othersize cylinder diameters may be utilized.

Next, inner cylinder 42 is placed on a mandrel and four to five layersof fine industrial grade glass strands 46, shown in FIG. 4, commonlyhaving a thickness of about 0.00015 inch, are tightly wrapped over theentire outer surface where insulation is required while at the same timeavoiding the interconnect areas of inner cylinder 42. The tight wrappingof multiple layers of glass fiber strands over outer surface of innercylinder 42 provides structural support for the tubular structure. Theglass fiber layers also provide a certain degree of physical separationand concomitant electrical insulation between inner cylinder 42 andouter cylinder 44. An exemplary illustrative thickness of the glassfiber layers is about 0.00075 inch and is therefore relatively small butmay add significant strength. The wrapped inner cylinder 42 is theninserted all the way into outer cylinder 44 so as to ensure concentricand axial alignment of both cylinders and matching of respective holeson each side of inner cylinder 42 with the corresponding holes on eachside of outer cylinder 44 (FIG. 4). The next step is to tightly wraplayers of industrial grade glass fiber strands over the outer surface ofouter cylinder 44 in the same way as was done with inner cylinder 42.This glass fiber layer provides for structural support. An exemplaryillustrative thickness of the outer cylinder glass fiber layers is about0.001 inch. The electrical insulation and stator coil structuralstrength required depends on the application of the brushless motorbeing produced.

The matched holes are utilized to provide solder flow paths tointerconnect pads of each coil segment using for example alead-silver-tin solder material which can withstand operationaltemperatures as high as 450 degrees Fahrenheit (“F”). This interconnectcan be welded instead of soldered to create an interconnect with copperas the base weld material to allow even higher stator coil temperatures.Alternative methods of joining the matched holes may be used, such ascrimping, spot welding or laser welding. If welding is used, the statorcoil operational temperature may rise to about 600 degrees F., which isthe utilization temperature of the encapsulation material to be appliedlater. The matched solder holes (See FIGS. 1A and 1B) e.g., 34, 36, and40, 38, respectively, are not required if solder is not the selectedbonding material.

The soldered joints electrically interconnect all outer cylinder 44conductive bands with respective inner cylinder 42 conductive bands soas to form a continuous, inductive helical structure as shown in FIG. 5.FIG. 5 shows an exemplary induction coil on a brushless motor statorcoil, illustrating in detail how a portion of the helical structure ofthe coil winding can be accomplished with one end A of the winding beingon the inner cylinder and the opposing end A′ of the winding being onthe outer cylinder. For example, inner cylinder 42 conductive band 23 iselectrically connected at one end (hole 33) with outer cylinder 44conductive band 19 and at the other end (hole 41) with outer cylinderconductive band 21. The rest of the inner cylinder 42 conductive bandsare similarly interconnected with respective outer cylinder 44conductive bands with the total number of interconnections at each endbeing the same. Essentially, the inner cylinder 42 conductive bandsprovide one half of the electric circuit and the outer cylinder 44conductive bands provide the other half of the circuit. Joining the twohalves completes the electric circuit. In comparison with a wire woundstator coil, the wire has a minimum bend radius at the cylinder endsthat increase the stator coil wall thickness.

In at least one embodiment of the brushless motor, the stator coil isconstructed with three separate windings to support three phase ACoperation. Alternatively, the stator coil can be configured to supportany AC brushless motor without departing from the inventive conceptsdescribed throughout. In three phase AC motor applications, theinterconnects at the start and finish of each of the three windings areleft unsoldered or unwelded for later connection as required by theparticular circuit design.

The assembled stator coil 47 can be impregnated with encapsulatingcompound to provide additional structural stability, to permanentlysecure all components, and to provide complete electrical insulation ofthe stator coil. Specifically, the stator coil 47 can be impregnatedwith encapsulating polyimide, for example, a polyimide comprised of 25%solid/solute (polyimide) and 75% solvent (NMP). Polyimides are known fortheir high thermal resistance and are also generally considered to benon-flammable due to their aromatic, halogen-free structure thatmanifests itself in a very high limited oxygen index (about 38%). Whensubjected to flame, polyimide has a very low level of smoke formationand toxic gas formation, which makes it a good bonding agent for thestator coil 47. Polyimide is also chemically resistant to organicsolvents such as alcohol, ketones, chlorinated hydrocarbons, and has lowmoisture absorption. The stator coil 47 can then be centrifuged,injected, dipped, impregnated or otherwise encapsulated to replace airvoids with the polyimide solution. Centrifugal force pushes the air outof the structure and pushes the polyimide deeper into the crevices andcracks of the telescoped tubular structure allowing permanent bondingand insulation of the components.

The polyimide impregnated stator coil 47 can be heat-cured for exampleat a temperature of about 500 degree F. to remove solvents and to yielda hardened, cured polyimide encapsulated stator coil. A limitation tothe curing temperature is the solder flow temperature generally about550 degree F.; however, using non-solder welding techniques may allowpolyimide curing at 695 degrees F. and stator coil operatingtemperatures of 600 degrees F. Other potting materials may be used suchas ceramic, glass, silicates, silicones, etc. After the stator coil hasbeen heat-cured, it is allowed to cool to room temperature. The endproduct is a strong, stiff and fully insulated stator coil that can beused in any motor application either as a stator coil in a brushlessmotor or an armature of an electromotive having brushes. An exemplarystator coil well suited for brushless motor applications is described inU.S. Pat. No. 6,111,329 entitled “Armature for an Electromotive Device,”assigned to the assignee of the present invention and incorporatedherein by reference.

It is worth noting that the cured stator coil 47 commonly has very lowmoisture absorption due to the composition of its components, which arecommonly copper, high-quality soldering material, glass fiber andpolyimide. This adapts the stator coil 47 for use in brushless motorsused in medical/dental applications inasmuch as repeated heatsterilization or autoclave runs will not affect the operation of thestator coil.

An exemplary brushless motor with a free-standing stator coil is shownin longitudinal cross-sectional view in FIG. 6. The stator is shownseparately in longitudinal cross-section in FIG. 7. The rotor is shownseparately in longitudinal cross-section in FIG. 8 and transversecross-section in FIG. 9. The brushless motor 60 includes a rotor havingan outer cylinder 61 fitted with a rotor cap 62 at one end. The outercylinder 61 can be an iron cylinder which provides a magnetic returnpath during motor operation. The rotor cap 62 can be configured tosupport a rotor shaft 63 extending through the outer cylinder 61 alongits central axis. Positioned between the outer cylinder 61 and the rotorshaft 63 is an inner cylinder 71 concentrically aligned with the outercylinder 61. The inner cylinder 71 provides a mounting surface for amagnet 65. This arrangement results in a magnetic air gap 120 betweenthe outer cylinder 61 and the magnet 65.

The magnet 65 can be implemented in a variety of ways. An exemplarymagnet 65 is shown in transverse cross-section in FIG. 13 with thestator coil 47 (heavy dotted line) disposed within the magnetic air gap120 between the outer cylinder 61 and the magnet 65. The magnet 65 isshown as eight separate magnets equally spaced apart along the,circumference the mounting surface of the inner cylinder 71 of therotor. As those skilled in the art will appreciate, the number ofmagnets and their respective arrangement on the mounting surface of theinner cylinder will vary depending on the number of poles and theoverall design constraints. The stator coil 47, indicated in heavydotted line, is spaced from the rotor and does not make contact eitherthe magnet 65 or the outer cylinder 61 of the rotor. The rotor is,therefore, able to rotate freely when the stator coil 47 is disposedwithin the magnetic air gap 120 between the magnet 65 and the outercylinder 61 of the rotor. The individual magnets may be bonded to themounting surface of the inner cylinder 71 as shown in FIG. 13, oralternatively, to the interior surface of the outer cylinder 61.

The stator includes a stator faceplate 64 for supporting the stator coil47. Extending from the stator faceplate through the center of the statorcoil 47 is a hollow shaft 66. The hollow shaft 66 can be equipped withfront and rear ball bearings 67 and 67′, respectively, for rotatablysupporting the rotor shaft 63. The rotor shaft 63 projects forward ofthe stator faceplate 64 and is available for performing mechanical work.

In an exemplary embodiment of a three phase AC brushless motor, threesensors 68, such as Hall effect transistors, can be rigidly affixed tothe stator coil 47 and the stator faceplate 64 radially separated fromone another by 120 electrical degrees. Only one of the three sensors isshown in FIG. 6. The sensors sense the position of the magnets and sendsa signal to a controller (not shown in FIG. 6) that is used for timingthe switching and routing of electrical current to the respectivewindings of the stator coil 47. The timing of the change in current flowin the respective windings in the stator coil, triggered in response tothe position of the magnets, is such as to produce a tangential forcecausing the rotor shaft 63 to rotate. These techniques are well known inthe art.

The brushless motor can be used in a variety of applications such asdrills (dental, medical, commercial), hobby craft, automotive,aerospace, photocopiers, printers, robotics, disc drives, pumps,compressors and motion control devices. In addition, the rotor (and themagnet affixed thereto) can be disposed to either underlie or overliethe stator. In an outer rotor embodiment (i.e., with the rotor overlyingthe stator) may be used, for example, for a belt drive. Theelectromotive device may be used for electrical generation and eddycurrent dampers in the same configuration.

While particular embodiments of the present invention have beenillustrated and described, it would be apparent to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. For example, thebrushless motor in alternative embodiments may be configured to provideelectrical generation when the shaft is rotated by mechanical means.Further, the iron ring may not be required for some applications and thestator can be formed into a cartridge which can be inserted into theinner diameter of a fixed laminate stack. It is therefore intended tocover in the appended claims all such changes and modifications that arewithin the scope of this invention.

1. A brushless motor, comprising: a rotor having a magnet and a magneticreturn, the magnet and magnetic return being arranged to form a gaptherebetween; and a stator having a coil disposed in the gap.
 2. Thebrushless motor of claim 1 wherein the rotor further comprises a shaftrotatably coupled to the stator.
 3. The brushless motor of claim 1wherein the rotor further comprises a rotor faceplate supporting themagnet and the magnet return.
 4. The brushless motor of claim 3 whereinthe rotor further comprises a mounting surface supporting the magnet. 5.The brushless motor of claim 4 wherein the magnet comprises a pluralityof magnets.
 6. The brushless motor of claim 3 wherein the rotor furthercomprises a shaft coupled to the rotor faceplate.
 7. The brushless motorof claim 1 wherein the stator coil comprises a pair of concentricconductive sheet metal windings each comprising a plurality of axiallyextending spaced apart conductive bands, each of the conductive bands ofone of the windings being coupled to one of the conductive bands of theother winding.
 8. The brushless motor of claim 7 wherein the statorfurther comprises a stator faceplate supporting the stator coil.
 9. Thebrushless motor of claim 8 wherein the rotor further comprises a shaftrotatably coupled to the stator faceplate.
 10. A brushless motor,comprising: a rotor having a mounting surface, a magnet disposed on themounting surface, and a magnetic return surrounding the magnet to form agap therebetween; and a stator having a coil disposed in the gap. 11.The brushless motor of claim 10 wherein the mounting surface and themagnetic return are concentric.
 12. The brushless motor of claim 11wherein the mounting surface and the magnetic return are cylindrical.13. The brushless motor of claim 12 wherein the stator coil iscylindrical and is disposed in the gap concentrically with the mountingsurface and the magnetic return of the rotor.
 14. The brushless motor ofclaim 10 wherein the magnet comprises a plurality of magnets eachextending along the mounting surface in an axial direction and beingspaced apart an equal distance from one another.
 15. The brushless motorof claim 10 wherein the stator further comprises a stator faceplatesupporting the stator coil, the rotor being rotatably mounted to thestator faceplate.
 16. The brushless motor of claim 15 wherein the rotorfurther comprises a shaft supported by the rotor faceplate, the shaftbeing rotatably mounted to the stator faceplate.
 17. The brushless motorof claim 10 wherein the rotor further comprises a rotor faceplate withthe mounting surface and the magnetic return concentrically extendingtherefrom in an axial direction, and a shaft extending axially along acentral axis common to the mounting surface and the magnetic return, andwherein the stator further comprises a stator faceplate supporting thestator coil, the rotor shaft being rotatably to the stator faceplatesuch that the stator coil can rotate in the gap.
 18. The brushless motorof claim 17 wherein the mounting surface, the magnetic return, and thestator coil are cylindrical.
 19. The brushless motor of claim 10 whereinthe stator coil comprises a pair of concentric conductive sheet metalwindings each comprising a plurality of axially extending spaced apartconductive bands, each of the conductive bands of one of the windingsbeing coupled to one of the conductive bands of the other winding.
 20. Abrushless motor, comprising: a rotor having first and second concentriccylinders, and a magnet disposed on an outer surface of the firstcylinder, the second cylinder surrounding the first cylinder to form acylindrical gap therebetween; and a stator having a free-standingcylindrical coil disposed in the gap.
 21. The brushless motor of claim20 wherein the stator coil comprises a pair of concentric conductivesheet metal windings each having a plurality of axially extendingconductive bands separated from an adjacent conductive band by a space,each of the conductive bands of one of the windings being coupled to oneof the conductive bands of the other winding.
 22. The brushless motor ofclaim 20 wherein the rotor further comprises a circular disc, the firstand second cylinders extending from the circular disc in an axialdirection.
 23. The brushless motor of claim 22 wherein the rotor furthercomprises a shaft coupled to the circular disc and extending therefromin an axial direction along a common axis of the first and secondcylinders, the shaft being rotatably coupled to the stator.
 24. Thebrushless motor of claim 23 wherein the stator further comprises afaceplate supporting the stator coil, the rotor shaft being rotatablycoupled to the stator faceplate.
 25. The brushless motor of claim 21wherein the magnet comprises a plurality of magnets each extending in anaxial direction along the outer surface of the first cylinder and beingequally spaced apart from one another.